Is there a problem being a Mason?

If he plans to abandon Freemasonry for Catholicism, yes, of course, he can become Catholic. But Catholics are prohibited from involvement in Freemasonry, so if he intends to remain a Freemason after becoming a Catholic, to my knowledge, he cannot enter the Church. The Church has long stated that:

Therefore, the Church’s negative judgment in regard to Masonic associations remains unchanged since their principles have always been considered irreconcilable with the doctrine of the Church and, therefore, membership in them remains forbidden: The faithful who enroll in Masonic associations are in a state of grave sin and may not receive holy Communion (Declaration on Masonic Associations).